This invention relates to production of high purity product water, and more particularly it relates to a method and apparatus for producing product water having a high level of electrical resistivity.
One of the problems in producing high purity product water in a double pass reverse osmosis is that it is difficult to reject gases such as carbon dioxide and/or ammonia because such gases are not easily removed by reverse osmosis membranes. The carbon dioxide and ammonia gas pass through the reverse osmosis system and re-establish an equilibrium in the product water and adversely affect product water resistivity. However, merely controlling the gases in the feedwater does not ensure high resistivity water.
The pH of feedwater to a double pass reverse osmosis is often controlled to provide high resistivity water. However, often, a pH range of feedwater that produces high resistivity product water in one instance may not always produce high resistivity product water in another instance. That is, pH of feedwater to the first pass reverse osmosis does not always provide a control that produces high resistivity product water.
The presence of total alkalinity due mainly to bicarbonate, smaller amounts of carbonate, with small contributions by other ions and of carbon dioxide in the feedwater is responsible for significant changes in apparent rejection of salts and thus in the conductivity of product water from a double pass reverse osmosis system. As noted, reverse osmosis membranes are transparent to dissolved gases. Thus, CO.sub.2 present in the feed side of the first pass membrane passes through the membrane to the interpass while bicarbonate and carbonate comprising total alkalinity is mostly rejected along with other anions and cations. This results in a change in the total alkalinity: CO.sub.2 ratio, a loss of buffering capacity and causes a drop in pH from feed to interpass or permeate from the first membrane in a double pass reverse osmosis system. The same process is repeated from the interpass to the second pass product. The resulting change in the interpass pH can have the result of moving the interpass pH away from the pH which results in high resistivity product water from the second pass reverse osmosis unit. Thus, setting the pH of the feedwater to a double pass reverse osmosis does not always result in high resistivity product water.
In addition, when a particular pH is chosen for producing high resistivity water from a particular feedwater, changes in the feedwater composition, e.g., alkalinity, can render the chosen pH not optimum. Thus, lower quality product water results even though the feedwater has been maintained within a narrow pH range which was, at one time, thought to be optimum. Further, it will be appreciated that differently charged membranes have the capacity to reject different ions to a lesser or greater extent. That is, positively charged membranes reject anions better than cations and vice versa for negatively charged membranes. When there is preferential rejection, there can be leakage of the other or opposite ion. pH and ionic strength of the feedwater has a large impact on the capacity of the particular membrane to reject the particular anion or cation. However, any charged membrane's performance can vary in a systematic way with pH to reach a peak value for rejection, and thereafter its performance declines on either side of an optimum pH. This effect is very significant at the lower ionic strength prevailing in the feed to the second pass.
This concept is illustrated in FIG. 3 where A and B denote the highest resistivity for a given pH value on two different waters. On either side of these points, resistivity declines. FIG. 3 also illustrates that two different pH values can result in the same quality product water. However, on either side of a certain pH value, product quality declines. "A" may can represent low alkalinity and TDS and "B", high alkalinity and TDS. Further, the process is complicated by membrane selection. The negatively charged membrane of Fluid Systems Inc., referred to by the tradename HRRX membrane, operates in a pH range of 6.5 to 8 with a maximum 99.4% rejection, while Toray's positively charged membrane, having the designation SU910S, operates at a pH of 9 to 9.5 with a maximum 99.5% rejection. Lower pH is better for removing ammonia and higher pH is better for removing carbon dioxide. Thus, it will be seen that there is a great need for a process which can be tuned to the system, including feedwater and the use of different membranes, and which will consistently produce high resistivity water on a continuous basis, even with changing composition of the feedwater.
Attempts at removing carbon dioxide to provide high resistivity water in the past have only been partially successful and often end up further contaminating the water. For example, U.S. Pat. No. 4,574,049 discloses a process for removing carbon dioxide and other impurities from a water supply using double pass reverse osmosis membranes. The process includes providing a first reverse osmosis until having an inlet, a product outlet and a brine outlet; providing a second reverse osmosis unit having an inlet, a product outlet and a brine outlet; locating the second reverse osmosis unit downstream of the first reverse osmosis unit with the product outlet of the first reverse osmosis unit being coupled to the inlet of second reverse osmosis unit; providing water to be purified to the inlet of first reverse osmosis unit; treating the product from the reverse osmosis unit at a location upstream of second reverse osmosis unit with a chemical treatment agent comprising a solution having a pH that exceeds 7 to reduce carbon dioxide concentration of the product by chemical conversion and to ionize certain otherwise difficult to remove chemicals; and directing the product from second reverse osmosis unit toward a point of use or storage for purified water.
However, this process which normally uses sodium hydroxide for increasing the pH results in the addition of sodium which, because of its small ionic radius, is difficult to remove by subsequent membranes. Further, the addition of sodium hydroxide has another disadvantage in that the series of reactions removing carbon dioxide are relatively slow when compared to reverse osmosis unit contact time. Thus, the effectiveness of the operation is limited by the sodium hydroxide reactions, and further, this process does not remove ammonia.
U.S. Pat. No. 5,338,456 discloses a water purification process for removing dissolved solids of the type that are normally present in a municipal or similar water supply. The process uses a forced draft decarbonator having an inlet and a product outlet, a vacuum degasifier having an inlet, a product outlet and a water level sensor, and a reverse osmosis unit having an inlet, a product outlet and a brine outlet. The vacuum degasifier is located downstream of the forced draft decarbonator with the product outlet of the forced draft decarbonator being coupled to the inlet of the vacuum degasifier. The reverse osmosis unit is located downstream of the vacuum degasifier with the product outlet of the vacuum degasifier being coupled to the inlet of the reverse osmosis unit. Water to be purified is provided to the inlet of the forced draft decarbonator at a predetermined rate. According to the invention, the rate at which water to be purified is provided to the inlet of the forced draft decarbonator is a function of a predetermined water level in the vacuum degasifier.
Japanese Patent 4-22490 discloses a pre-stage reverse osmosis membrane module, a post-stage reverse osmosis membrane module and a hydrophobic porous membrane module, to which an aqueous alkali solution circulating line is attached in the permeate side. That is, Japanese Patent 4-22490 utilizes an alkali solution in the permeate side to remove dissolved carbon dioxide by chemical reaction. The hydrophobic porous membrane module is placed between the pre-stage module and the post-stage module and has pores capable of permeating only gases. An inert gas blowing pipe is installed to the alkali aqueous solution circulating line.
Japanese Patent 2-2802 discloses reverse osmosis separator membrane module and degassing membrane module arranged in treating water line in series. The degassing membrane is formed by a porous supporter layer and high molecular homogeneous layer or minute layer arranged on the supportor layer. Oxygen separating. coefficient of the degassing membrane is not less than 1.3.
U.S. Pat. No. 4,897,091 discloses that gases such as carbon dioxide may be separated from rich liquor (such as methanol containing carbon dioxide) by passage of gas through a membrane which is the reaction product of (i) a polyamine and (ii) a polyisocyanate or a poly (carbonyl chloride).
U.S. Pat. No. 5,078,755 discloses removing dissolved gas from liquid, which comprises bringing the liquid containing the gas dissolved therein into contact with a membrane, thereby causing the dissolved gas to selectively permeate the membrane. The membrane is a permselective, composite membrane composed of a porous support and a nonporous, active membrane of a synthetic resin formed on the porous support, or is a permeable membrane having such characteristics that the nitrogen gas permeation rate at 30.degree. C. is in the range from 7.times.10-4 to 2.times.102 Nm3m2.cndot.h.cndot.atom and that the amount of permeated stream is 100 g/m.sup.2 .cndot.h or less when 20.degree. C. water is supplied to the membrane under atmospheric pressure while maintaining the pressure on its permeate side at 40 mm Hg.
U.S. Pat. No. 5,106,754 discloses that total organic carbon (TOC) and total inorganic carbon (TIC) monitoring of water is useful in determining the water quality. Conventional TOC and TIC monitoring techniques are not zero gravity compatible. The addition of microporous hydrophobic bladders in combination with a non-dispersive infrared analyzer allow for a two-phase, liquid and gas, zero gravity compatible TOC monitoring technique.
U.S. Pat. No. 5,116,507 discloses a method of treating an aqueous liquor, such as effluent liquor formed during coal gasification. The method comprises subjecting the liquor to dephenolation and ammonia stripping treatment to remove phenolic compounds and "free" ammonia from the liquor and then subjecting the resulting liquor, which still contains ammonium compounds and thus "fixed" ammonia, to reverse osmosis treatment to produce a permeate which is substantially free from impurities, including fixed ammonia.
U.S. Pat. No. 5,250,183 discloses an apparatus for manufacturing ultra-pure water, characterized in that a decarbonator/degassor and a reverse osmosis equipment for pretreatment of supply water are installed in the upper stream of a multiple effect evaporator.
U.S. Pat. No. 5,254,143 discloses a diaphragm for gas-liquid contact comprising a membrane having two surfaces, at least one surface of the membrane is hydrophilic and surfaces of micropores present in the membrane are hydrophobic. The diaphragm is used in contact apparatus in which a liquid is contacted with the hydrophilic surface of the membrane and a gas is contacted with the other surface.
U.S. Pat. No. 5,306,427 discloses a process for the separation of one or more, more permeable components from one or more, less permeable components in a feed stream. The process suggests two membrane separation stages in series wherein the feed is introduced into the low pressure side of the first stage, the permeate stream from the first stage is compressed and introduced into the high pressure side of the second stage and wherein the non-permeate stream from the second stage is recycled to the high pressure side of the first stage.
U.S. Pat. No. 5,413,763 discloses a method and apparatus for the measurement of the total organic carbon (TOC) content of a liquid. The inorganic carbon in the liquid is converted into carbon dioxide and removed from it. At the same time, oxygen is added to the liquid. The liquid is then exposed to ultraviolet radiation and the organic carbon thereby oxidized.
Japanese Patent 4-176303 discloses a gas-permeable membrane module containing a hollow fiber-shaped hydrophobic gas-permeable membrane used to remove the gas dissolved in a liquid. The liquid is supplied from an inlet, passed through the inside of the membrane from the membrane opening and sent to the other end of the membrane. A carrier gas is introduced from an outlet, passed around the bundle of the membranes and discharged from an outlet. The outlet is connected under these conditions to a vacuum source such as a vacuum pump, hence the gas dissolved in the liquid permeates through the membrane to the outside, and degasification is performed with high efficiency.
In U.S. Pat. No. 5,156,739, it is disclosed that water to be purified and degassed is passed through a reverse osmosis step from which a pure water stream and a high pressure waste water stream are produced. The high pressure waste water is passed through an eductor to produce a vacuum. The pure water stream is passed into a first volume of a degassifier and the vacuum is directed to a second volume of the degassifier. The first and second volume of the degassifier are separated by a hydrophobic membrane.
In spite of these prior processes which focus on CO.sub.2, there is still a great need for an improved process which will produce high purity or high resistivity water on a continuous basis. Such a process should also take into account the phenomenon illustrated by FIG. 3.